Sky & Telescope - January 2019 - 26

The Next Gravitational-Wave Revolution
Lovell
CHIME

Westerbork
Effelsberg

Nançay
VLA
Green Bank

Sardinia
FAST

Arecibo

Parkes

MeerKAT

p WORLDWIDE EFFORT Astronomers use 12 radio observatories to track roughly 75 millisecond pulsars. Of these facilities, three - CHIME in
Canada, FAST in China, and MeerKAT in South Africa - are new participants; the others have been involved for at least 10 years.

ral in a galaxy such as the gigantic elliptical M87 would emit
detectable gravitational waves for 4 million years. But a humbler black hole pair in a smaller galaxy, such as the Sombrero
(M104), would offer a 160-million-year window. The odds
that a very massive binary is sending out gravitational waves
during the time that PTAs are observing is thus lower.
Unless we're really lucky, it will probably take 10 to
15 years to build up enough data to see deep enough into
space to detect an individual binary. Theorists expect the
gravitational waveform from a single binary to be a very simple sinusoid, and because the binary system will likely have
an orbital period of several decades, the signal will change
very little over many years.
The longer baseline could make electromagnetic follow-up
easier. If the inspiraling and merging black holes are embedded in disks of gas - and recent work suggests that this could
be true for supermassive binaries, unlike the smaller ones that
LIGO and Virgo detect - then they would also produce light.
"If a loud individual source is detected, PTAs will give its sky
localization, opening the possibility to identify the host galaxy
and carry out multimessenger observations of the system,
pretty much as it happened with the LIGO neutron star binary
GW170817," Sesana says (S&T: Feb. 2018, p. 32). "The difference is that with PTAs, everything builds up slowly over the
course of the years, and you don't hit the jackpot in a snap."

to shorter-wavelength gravitational waves. All of this works
to increase sensitivity to both the stochastic background and
individual binaries.
But progress has been slow for a variety of reasons, from
planning across time zones and work cultures to accounting
for each telescope's individual quirks. There's also the all-toohuman tension of each team wanting for itself the glory of
making the ﬁrst conﬁrmed detection.

2025
2035

2050

2000

2040
2005

Vernal
equinox

2015
2030

Sun

2010

Going Global
Adding to the growing optimism is the fact that scientists
from all three teams are now combining their data sets to
form an even more powerful network: the International Pulsar Timing Array (IPTA). The IPTA enables scientists to analyze data from most of the observed pulsars, including from
facilities in both hemispheres. Having more observations
means shorter time gaps in data, which increases sensitivity
26

JA N UA RY 2 019 * SK Y & TELESCOPE

2045

Now

2020

M A P: G REGG DINDER M A N / S&T; BA RYCEN TER:
L A RRY MCNISH / CA LG A RY CEN TER OF THE ROYA L
ASTRONOMICAL SOCIE T Y OF CA NADA

EPTA
NANOGrav
PPTA
New members

Sky & Telescope - January 2019

Table of Contents for the Digital Edition of Sky & Telescope - January 2019